Jim Bob1055

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Engine is idling low (650 rpm)-- rough and stalls.
Engine check light on with code P0300 only.
Changed the plugs and air-filter to no effect.
Even when the idle is held at 1000 rpm it feels a bit rough, but pulls cleanly and strong to red line.
From what I've read so far there are several other components that can cause rough idle (CPS, TPS, EGR).
What next and if a vacuum leak check is needed what method )of detection is best?